Altermagnetic Magnons in Dipolar Nanomagnet Arrays

Abstract

Altermagnetism is conventionally understood in a spin-conserving framework, where symmetry-enforced momentum-dependent spin splitting emerges in collinear magnets with vanishing net magnetization. Here, we show that its defining signatures persist in nanomagnet arrays coupled exclusively by dipolar interactions, despite the intrinsic breaking of spin conservation. Using a macrospin theory of dipolar-coupled ferromagnetic nanoislands, corroborated by micromagnetic simulations, we demonstrate that arrays engineered with altermagnetic symmetries exhibit spin-split magnon bands whose eigenstates can partially remain strongly spin polarized. The resulting spin expectation value displays the characteristic $d$-wave pattern throughout the Brillouin zone, establishing a mesoscopic realization of altermagnetic magnons beyond the conventional spin-conserving paradigm. As a consequence, spin-wave propagation becomes strongly direction dependent, providing a highly tunable platform for anisotropic magnon transport and synthetic altermagnetic functionality.